After injecting rats with a lab version of Botox, Italian scientists found evidence that the nerve toxin had migrated from the animals' whisker muscles into their brains.

If you got one of the 2.8 million cosmetic Botox procedures in the United States last year (at an estimated $380 apiece), this news could bring the frown lines right back.

Not to worry, say local docs who have read the results in the current Journal of Neuroscience.

It was well known that Botox works by entering the facial motor neurons and chopping up a protein needed to make muscles operate. These nerves extend back into the brain stem - so it was no surprise that researchers found the protein fragments there, says neurologist Stephen Silberstein.

More surprising to him: When those researchers injected the toxin directly into a rat's brain, there was evidence that it migrated from one hemisphere of the brain to the other.

"I think it's extremely interesting, but I think it's unproven," says Silberstein, director of the Jefferson Headache Center, who uses Botox to treat migraines.

Besides, Silberstein says, people don't get Botox injected directly into their brains, so the findings, even if replicated, are not likely to be relevant.

Still, it may be wise to "think twice" before getting the cosmetic injections, says S. Ausim Azizi, Temple University Hospital's neurology chair. If you must, get the smallest dose possible, he says.

(The researchers did not note that the treatment caused any change in the rats' behavior, though, as in humans, it did make their facial muscles relax.) - Tom Avril